Continuing advances in information handling technology have lead to the requirement for data storage and retrieval systems capable of handling extremely large volumes of information. Optical recording, in particular optical disc recording, allows recording and accessing of information at very high data rates with a much greater recording density and archivability than is possible with magnetic recording. A highly focussed laser beam is utilized to record and recover information on the optical recording media. The selection and alignment of diode lasers into an optical recording system is discussed by Bartolini et al. in I.E.E.E. Journal of Quantum Electronics, 1981, p. 69, and both read and write apparatus are disclosed in British Patent Application No. 1 016 747A.
Many types of recording media have been disclosed for laser writing and these can be divided into two basic sorts: those which require processing after writing, and those which can be read immediately after writing. It is the latter type, possessing "direct read after write" capability and commonly known as "DRAW" media, which are of particular interest.
In order to be useful as a light absorbing layer of the recording element, materials must be able to be applied to a substrate in the form of a thin, smooth layer of high optical quality and predetermined thickness and they must absorb at the frequency of the optical source. Various materials have been proposed for the recording media of DRAW systems, including, for example, thin metal films, metal-impregnated polymers and organic dyes. In these cases the laser beam provides a pulse of heat energy to the recording medium which causes a change in surface morphology; i.e., formation of a bump or crater, by ablation, vaporization or melting.
One type of DRAW media comprises thin metal films and, of these, tellurium containing mixtures as disclosed in Lou et al., J. Vac. Sci. Technol., 1981, 18, 78 have been widely used. However, the preparation of recording elements incorporating tellurium is by a relatively expensive vacuum sputtering technique in which the metal does not adhere well to the substrate. It also presents environmental complications because of its toxicity.
Examples of the use of metal-impregnated polymers in recording elements include the silver-impregnated gelatin systems disclosed in U.S. Pat. No. 4 278 758. Greater sensitivity is claimed for these systems than for the tellurium films, but high concentrations of expensive silver are used in the recording medium.
An alternative type of DRAW media uses organic compounds in place of expensive metals. As well as providing advantages of cost, the thermal properties of organic compounds are generally superior since they possess low thermal conductivity and low melting/decomposition temperature. With the use of such systems it is important that the absorption of dye therein corresponds as closely a possible with the emission of the recording laser. Of the various lasers available, semi-conductor laser diodes have the advantages, over conventional gas lasers, of low cost and size and the possibility of easy signal modulation. The problem is, therefore, one of finding organic materials which have all the requisite physical properties and absorb strongly in the region compatible with laser diodes; i.e., the near infrared region of the spectrum, wavelengths between 700 and 1400 nm. Examples of dye-containing recording media for optical data storage are disclosed in Jipson and Jones, J. Vac. Sci. Technol., 1981, 18, 105, European Patent Application No. 79 200 789, Crowly et al., IBM Technical Disclosure Bull, 24, No. 11B, 1982, Law et al., Appl. Phys. Lett., 1981, 39, 718, U.S. Pat. Nos. 4,270,130, 4,364,986 and 4,446,223, PCT Patent Publication Nos. WO84/02794 and WO84/02795 and Japanese Patent Publication Nos. 57 203 237, 57 210 893, 57 210 894, 58 053 489, 58 056 894, 58 056 895, 58 077 043, 043, 58 112 792, 58 219 090, 58 222 451, 58 224 447, 59 005 095, and 59 055 795.
The use of organic dyes in optical data storage systems has, however, some problems since the dyes tend to crystallize once applied to the substrate in thin layers, with an accompanying reduction in medium performance. This problem has been resolved, in the main part, by the coating of dye in a polymeric binder with a resultant elimination of significant detrimental crystallization.
Many of the reports on dye-in-binder systems have been concerned with the ablation properties of dye-polymer films.
In practice, the major problem encountered in the use of dye-polymer media is not that ablation does not take place, but rather that on being repeatedly read, particularly when using a "read laser" of the same wavelength as the "write laser", the carrier to noise ratio is unacceptably degraded. To reduce the cost of the write/read device it is conventional that only a single laser is used. Therefore although the read laser power used is only a fraction of the write power, it is inevitable that absorption of the incident light during reading will occur.
There is no specific teaching in the art as to the type of binder which is most suitable for use in dye-polymer optical recording media. In general dye-polymer ablative systems are said to be superior to absorbing metal systems because of their low conductivity, low decomposition temperature and low melting point. Polymeric binders with low melting points are exemplified, together with binders of low or oligomeric weights as imparting high or increased sensitivity to the media. However, examples of thermoplastics having higher glass transition temperatures, e.g. polyesters, polycarbonates, poly(N-vinyl carbazole) appear in general lists of possible binders. The most commonly used binders are cellulose derivative, especially nitrocellulose, thereby obeying the guidelines of low decomposition/melting point and low binder molecular weight.
We have now found that dye-polymer ablative systems having a binder based on poly(acenaphthylene) possess particularly advantageous properties particularly with regard to stability and repeated reading. In particular, binders in which at least 25% by weight of the binder comprises a polymer in which at least 50% of the repeating units have a common nucleus of the general formula: ##STR2## are suitable for use in the invention.